Use bitcoin::Amount and bitcoin::FeeRate

[luisschwab]

Closes #192

Changelog

## Added
- Add `deserialize_fee_histogram`

## Breaking 
- Use `Amount` and `FeeRate` on `MempoolStats.fee_histogram`
- `submit_package` takes in `FeeRate` for `maxfeerate` instead of `f64`
- `submit_package` takes in `Amount` for `maxburnamount` instead of `f64`
- `get_fee_estimates` returns `FeeRate` instead of `f64`

## Changed
- Update `test_feerate_parsing` with an up to date fee histogram

[oleonardolima]

tACK 291aa09

I think one point of attention would be that currently the FeeRate in get_mempool_stats is being floored (e.g [5.0072994, 50347] -> (FeeRate(1251), Weight(50347))).